Considering half of our collection is Gottlieb...
Waterworld: is tons of fun. Sinking the ship has yet to get boring.
Stargate: has so much going for it.
Black Hole: is difficult, but keeps you coming back to get that multiball.
Hot Shots: is an amazing simple game, but getting those 15 drops down is one hell of a task. That keeps me coming back. Only done it twice on one ball.
Target Alpha: is classic and the most useless upper left flipper in probably all of pinball :p
Cactus Jacks: is light-hearted and good fun German dancing cowboy cacti. Not sure what more you're looking for.
I would love to play more of Gottliebs system 3 since they are BULLETPROOF.